1. Field of the Invention
This invention relates to tunable vacuum ultraviolet (VUV) sources of coherent radiation in the 150 nm spectral range, and more particularly to stimulated anti-Stokes Raman lasing from an inverted metastable population in atomic bromine.
2. Description of the Prior Art
In recent years tunable sources of incoherent as well as coherent radiation in the VUV spectral range have been developed.
One such method as disclosed in Harris et. al. in their article entitled "Anti-Stokes Emission as a VUV and Soft X-Ray Source", published in the book Picosecond Phenomena, edited by Schank et. al., Springer-Verlag, Berlin, N.Y., 1978. Harris et. al. describe spontaneous Raman emission of incoherent radiation from the 2s .sup.1 S excited state of He using the principle of four-wave parametric mixing. The 2s .sup.1 S He metastable state was produced and inverted by glow discharge. A Nd:YAG laser operating at 1.064 .mu.m produced Stokes and anti-Stokes spontaneous emission at 63.7 nm and 56.9 nm, respectively. They also suggest using the above He Raman flashlamp source to remove inner shell electrons in K in either one step or in two steps in order to produce excited states of K.sup.+ from which lasing may be induced, such as the 60.1 nm line of K.sup.+.
Druet et. al. in their paper entitled "Electronic Resonance Enhancement of Coherent anti-Stokes Raman Scattering", published in Physical Review A, Vol. 18, p. 1529, October 1978, give a theoretical treatment of coherent anti-Stokes stimulated Raman scattering involving two photon absorption from the exciting laser beam. a metastable state is produced and a pump beam is tuned near a dipole absorption transition from the metastable level to a virtual intermediate level. Druet et. al. provide theoretical background in the resonantly enhanced coherent anti-Stokes Raman process.
Wilke et. al. in their article "Tunable Coherent Radiation Source Covering a Spectral Range from 185 to 800 nm", published in the journal Applied Physics, Vol. 18, p. 177, 1979, disclose the use of nonresonant stimulated Raman anti-Stokes upconversion using the vibrational levels of molecular hydrogen. They used H.sub.2 gas and a variety of tunable dye lasers to generate anti-Stokes coherent Raman radiation up to 185 nm. Wilke et. al. make use of a four-wave mixing process for the anti-Stokes Raman upshift and do not produce an inverted metastable population. The four-wave mixing process used by Wilke et. al. makes use of the nonlinear properties of the medium. No metastable population inversion is produced. Output anti-Stokes light is produced only in the forward direction, and then only along phase matched cones. In four-wave mixing, the process depends upon the nonlinear susceptibility of the material.
White et. al. in an article "Anti-Stokes Raman Laser", published in Physical Review A, Vol. 25, p. 1226, February 1982, disclose a Tl vapor resonantly enhanced stimulated Raman anti-Stokes laser. They populated the 6p .sup.2 P.sub.3/2 .degree. level of Tl selectively with respect to the corresponding 6p .sup.2 P.sub.1/2 .degree. ground state by photodissociation of TlCl, and obtained stimulated anti-Stokes Raman emission from the .sup.2 P.sub.3/2 .degree. level using pump beams of both the second and third harmonics of a Nd:YAG laser. They provided anti-Stokes Raman output of 376 nm and 278 nm by tuning the pump beam for resonant enhancement by an intermediate dipole transition.
Loree et. al., in U.S. Pat. No. 4,144,464 issued March 1979, teach an anti-Stokes four-wave mixing device using energy levels of H.sub.2, D.sub.2, HD, CH.sub.3, and N.sub.2.
In none of the prior art has the problem of providing a high power tunable laser which provides VUV radiation in the 150 nm range been solved.